1. Field of the Invention
The present invention relates to semiconductor devices.
2. Description of the Related Art
In recent years, electronic circuits (hereinafter referred to as functional circuits) which are formed over substrates and have specific functions have been utilized for a variety of electronic components or semiconductor devices such as electronic devices.
As functional circuits, photoelectric conversion circuits can be used, for example. A large number of semiconductor devices having photoelectric conversion circuits generally used for detecting electromagnetic waves (also referred to as photoelectric conversion devices) are known. For example, semiconductor devices having sensitivity to ultraviolet rays to infrared rays are generally referred to as optical sensors. Among optical sensors, an optical sensor having sensitivity to a visible light region with a wavelength greater than or equal to 400 nm and less than or equal to 700 nm is particularly referred to as a visible light sensor. A large number of visible light sensors are used for devices which need illuminance adjustment, on/off control, or the like depending on human living environment.
A semiconductor device having a functional circuit, for example, the above photoelectric conversion device, has a problem in that an element included in the circuit is damaged when high voltage which is higher than certain voltage is applied from the outside. For example, in the case of a photoelectric conversion device, it has a problem in that electrostatic discharge (ESD) occurs in a photoelectric conversion circuit when high voltage of several kilo volts is applied to the photoelectric conversion circuit by static electricity or the like. With respect to the above problems, a semiconductor device which has a function of suppressing damage to an element even if high voltage that might damage a functional circuit is applied from the outside by providing a protection circuit to lower voltage applied to the functional circuit has been proposed (for example, Reference 1: Japanese Published Patent Application No. 2003-303890).